The invention relates to a demodulation unit for demodulating a quadrature input signal having at least two components, comprising a controlled oscillator for supplying a quadrature oscillator signal in dependence upon a control signal, and a mixer for mixing the quadrature input signal with the quadrature oscillator signal from the controlled oscillator and for supplying at least two mixed components, the mixer and the oscillator being incorporated in a PLL.
The invention also relates to a divider and a mixer for use in such a demodulation unit and to a receiver provided with such a demodulation unit.
The invention further relates to a method of demodulating a quadrature input signal having at least two components, wherein a quadrature oscillator signal is supplied in dependence upon a control signal and the quadrature input signal is mixed with the quadrature oscillator signal, and wherein at least two mixed components are supplied.
Such a demodulation unit and method may be used for, for example demodulating both AM and FM-modulated signals for radio, television, and communication receivers, etc., and for, for example "zero-IF" generation for QAM/QPSK signals and VSB signals, in which the PLL then comprises, for example, a COSTAS loop.
In addition to a synchronous demodulation function, such a demodulation unit also has a frequency-shift function.
In the present invention, a quadrature signal is also understood to be a real signal, which is a simplification of the signal to one of the two axes (X,Y) of the quadrature signal, in which both components are either equal or one of the components is zero.
A demodulation unit of this type is known from European patent application EP-A 0 579 100. This application describes a demodulation unit which comprises a PLL with a quadrature mixer and a controlled oscillator. This demodulation unit is intended to regain the phase and amplitude information of the carrier from the incoming signal.
A drawback of such a demodulation unit is that it is not very well possible to demodulate both FM and AM signals without taking extra measures as regards filtering, etc. Such a demodulation unit may also have acquisition problems, inter alia, due to amplitude variations.
To eliminate amplitude variations of the input signal, an extra bandpass filter and a limiter will have to be used for generating the carrier signal by means of the PLL. When the amplitude of the incoming signal varies, the lock-in range of the PLL will vary. Moreover, an amplitude variation of the control signal results in phase modulation of the carrier signal, which will distort the demodulated output signal.
If the PLL is used as an FM demodulator (for example, for TV sound), the amplitude variation of the input signal will result in a distortion of the demodulated output signal.
In a known digital or analog PLL, the limiter and filtering function must be incorporated in the carrier-generating circuit for demodulating AM signals. When demodulating FM signals, the limiter precedes the PLL so as to eliminate unwanted amplitude variations.